EVERETT — Boeing’s development of a new aerial-refueling tanker for the U.S. Air Force is “generally on track,” according to a report from the U.S. Government Accountability Office, as it starts a fast-paced flight testing schedule with little margin for delays or other problems.

However, the airplane maker could have to pay for projected cost overruns, which the Air Force estimated to be as much as $787 million, according to the report, which was released last week.

Boeing’s own estimate put it at $271 million.

Either way, taxpayers won’t have to pay unless the Air Force changes the contract requirements for the tanker, dubbed the KC-46A Pegasus.

The contract is worth about $4.4 billion and covers the plane’s development and delivery of the first 18 aircraft.

The Pegasus is a military version of Boeing’s 767-200ER commercial jetliner and will be assembled at the Chicago-based company’s Paine Field plant. Four test planes are already in various stages of production there.

Previously, the company had said flight tests would start in June. But on Friday, a company spokesman would only say that test flights will start this summer.

The program is on schedule to meet the contract’s timeline, said Jerry Drelling, a spokesman for the KC-46A program.

However, Boeing has used most of the program’s $354 million management reserve — money set aside to keep things on schedule when unexpected problems arise.

While not a flashing red light, “it is something to keep an eye on, because it sometimes indicates problems down the road,” said Michael Sullivan, the GAO report’s author.

If Boeing keeps using the reserve at its current rate of $9 million a month, the fund will run dry in September, well before the program has finished test flights.

Typically, this spending flatlines as development progresses, Sullivan said.

Air Force officials said in a congressional committee hearing earlier this month that the program is on schedule.

The tanker’s development has had far fewer problems and hiccups than most advanced military systems, Sullivan said.

The airplane’s software systems have been the program’s biggest source of problems. But those are a small fraction of software problems encountered when developing a new fighter aircraft, he said.

Boeing is using a lot of existing software — 83 percent — in the tanker. About 12 percent is modified from existing programs and only 5 percent is actually new software created for the KC-46A, according to the report.

That is part of the military’s effort to use more existing technology in new development programs, said Ed Gulick, an Air Force spokesman.

Part shortages set back Boeing’s schedule for making a fueling boom to use during testing, the report said.

Such delays are not unusual when developing a new military system, Sullivan said.

A boom is essentially a straw for transferring fuel from an aerial tanker to another airplane looking to gas up. The boom is lowered from the tanker’s tail to the other aircraft. Once connected, the fuel starts flowing.

The KC-46A will also be able to pass fuel to aircraft through hoses trailed from each wingtip, as well.

The tanker is designed to carry out multiple missions, including air refueling and transport of patients, passengers and cargo.

That flexibility means U.S. military commanders “won’t hesitate to use it in any theater of operations — worldwide — where we are tasked to support U.S. national security operations,” Lt. Col. Kathryn Barnsley, an Air Force spokeswoman, said in an email to the Herald earlier this year.

The tankers “will allow us to continue to project U.S. national security interests around the globe 24 hours a day with greater reliability and with increased capability,” she said.

The Air Force has ordered 179 KC-46A tankers from Boeing, which is scheduled to deliver the first 18 combat-ready planes by 2017. It will deliver the last six tankers in 2027.

The Air Force is expected to decide early in the next decade whether to order more KC-46s or another plane as it continues overhauling the tanker fleet, which currently relies largely on planes 30 to 50 years old.

Dan Catchpole: 425-339-3454; dcatchpole@heraldnet.com; Twitter: @dcatchpole.